package suanfa;

import java.util.ArrayList;
import java.util.List;

public class countSmaller {
    int[] ret;
    int[] index;
    int[] tmpIndex;
    int[] tmpNums;

    public List<Integer> countSmaller(int[] nums) {
        int n = nums.length;
        ret = new int[n];
        index = new int[n];
        tmpIndex = new int[n];
        tmpNums = new int[n];

        for (int i = 0; i < n; i++) {
            index[i] = i;
        }

        mergeSort(nums, 0, n - 1);

        List<Integer> l = new ArrayList<>();
        for (int x : ret) {
            l.add(x);
        }
        return l;
    }

    public void mergeSort(int[] nums, int left, int right) {
        if (left >= right) return;

        int mid = (left + right) / 2;
        mergeSort(nums, left, mid);
        mergeSort(nums, mid + 1, right);

        int cur1 = left, cur2 = mid + 1, i = 0;

        while (cur1 <= mid && cur2 <= right) {
            if (nums[cur1] <= nums[cur2]) {
                tmpNums[i] = nums[cur2];
                tmpIndex[i++] = index[cur2++];
            } else {
                ret[index[cur1]] += right - cur2 + 1;
                tmpNums[i] = nums[cur1];
                tmpIndex[i++] = index[cur1++];
            }
        }

        while (cur1 <= mid) {
            tmpNums[i] = nums[cur1];
            tmpIndex[i++] = index[cur1++];
        }

        while (cur2 <= right) {
            tmpNums[i] = nums[cur2];
            tmpIndex[i++] = index[cur2++];
        }

        // 修复：循环应包含 right，且索引偏移正确
        for (int j = left; j <= right; j++) {
            nums[j] = tmpNums[j - left];
            index[j] = tmpIndex[j - left];
        }
    }
}
